1. Field of the Invention
This invention relates to metal hydrides and in particular to processes involving such hydrides.
2. Art Background
Metal hydrides are used in a variety of industrial applications. Although there are many such applications, possibly the most prominent is the use of metal hydrides in batteries. For example, secondary nickel-metal hydride batteries employ lanthanum nickel hydride (or alloy modifications) or other intermetallic hydrides in the negative electrode. A variety of other uses involving energy storage and transfer have been described. Irrespective of the application, a crucial step in preparation is activation of the intermetallic. Activation is achieved, for example, by repeatedly reducing the metal such as LaNi.sub.5 to the corresponding hydride with H.sub.2 gas at high pressure and/or temperature followed by removal of hydrogen at lower pressures.
This cyclic process, generally denominated activation, is believed to serve two purposes. Each reduction to the hydride 1) removes reducible surface oxides which tend to interfere with the functioning of the material in the ultimate desired application, and 2) a reduction in particle size resulting from an increase in volume that causes fracture of the metal particles. Thus, activation, it is believed, removes oxide and produces in the hydride a greater surface area than was present in the corresponding metal. Both the removal of the oxide and the increase in surface area enhances the operation of the material for applications such as batteries where a surface oxide and a low surface area reduce the rate of the hydriding reactions.
Activation of hydrides has typically been performed at relatively high pressures (up to 1000 psi) and temperatures as high as 450.degree. C., by subjecting the metal directly to hydrogen gas. Clearly, although such conditions are not prohibitive to commercial use, they require relatively large expenditures for suitable equipment. Thus, an alternative to high pressure reaction of hydrogen gas with the corresponding metal would be quite desirable.